METHOD FOR DISTINGUISHING SECRETORY GRANULES OF DIFFERENT AGES - The present invention relates to an in vitro method of detecting secretory granules (SGs) of different ages in a cell by differentially labelling said SGs according to their age comprising (a) contacting a cell capable of forming SGs and expressing 1. a (poly)peptide specific for SGs, wherein said (poly)peptide has a binding site common for at least three different substances A, B and C covalently binding thereto or 2. a fusion protein comprising (i) a (poly)peptide specific for SGs and (ii) a (poly)peptide having a binding site common for at least three different substances A, B and C covalently binding thereto, wherein said substances are capable of penetrating the cell membrane and wherein at least substances A and C are detectable by different means with a substance A targeting said binding site (b) saturating said binding sites by contacting the cell with a substance B which blocks the binding sites which were not bound by substance A, (c) allowing that unbound substance B is removed from the cell or removing unbound substance B from the cell, (d) contacting the cell with a substance C targeting said binding site on the (poly)peptide or the fusion protein becoming available for binding after step (b) and (e) detecting for the presence of both detectable substances A and C, wherein the simultaneous detection of both substances A and C is indicative of the presence of two populations of SGs having a different age. Further, the present invention relates to an in vitro method of investigating in a cell the effect of a stimulus on secretory granules (SGs) differentially labelled according to their age.